Morbid and childhood obesity are characterized by excessive accumulation of adipose-tissue mass that results from enlargement of existing differentiated adipocytes (hypertrophic obesity) and from acquisition of new adipocytes from proliferation and subsequent differentiation of preadipocytes (hyperplastic obesity). Considering the rapid rise in childhood obesity and the devastating long term heath implications to these individuals, it has become critically important to gain greater knowledge of the molecular mechanisms regulating adipocyte hyperplasia. For several years, our lab has focused on the regulation and function of cyclin-dependent kinase (CDK) inhibitors, such as p27 and p21 that control G1 to S phase progression of the cell cycle as molecular regulators of preadipocyte replication. Preliminary data of this proposal demonstrate a physiological roll for the SCFSkp2 E3 ligase in mediating polyubiquitylation and degradation of p27 by the 26S proteasome during adipocyte hyperplasia. Furthermore, our preliminary data demonstrate that Cul1, a protein subunit of the SCF E3 ligase, is modified by the ubiquitin-like molecule Nedd8 in adipocytes and that the extent of Nedd8-Cul1 conjugates is regulated during G1 phase progression under physiological conditions. Based on these data and published literature, we developed the central hypothesis that Nedd8 modification of the SCF E3 ligase is a dynamic process that involves opposing pathways that attach and remove Nedd8 from its cullin protein target and that both pathways are essential for ubiquitin-mediated p27 protein degradation and adipocyte hyperplasia. We will test the central hypothesis in two aims. The first explores the physiological significance and mechanism of regulated Nedd8 modification during G1 phase progression regarding E3 ligase activity. The second determines the role of the CSN as a Nedd8 isopeptidase capable of removing Nedd8 and the impact regarding cellular SCF activity. Both aims address mechanisms linking attachment and removal of Nedd8 through evaluation of testable working models. We predict that pathways responsible for adding and removing Nedd8 are essential for SCF E3 ligase activity, p27 ubiquitylation and degradation, and adipocyte hyperplasia.